scheduler: Clean up priority/ratio task dead code
Since [1], tasks with priority are routed to dedicated queues (see tasks for more details). The tasks with priority to be scheduled have their own dedicated endpoints to be called.
- [1] Related to swh/infra/sysadm-environment#3084 (closed)
Related to #3271 (closed)
Test Plan
tox
Migrated from D5552 (view on Phabricator)